The electrical response to O-3 of 150-nm-thick carbon nanotube (CNT) thin films prepared by radio frequency-plasma enhanced chemical vapor deposition has been investigated at different operating temperatures starting from the room temperature. The interaction between ozone molecules and a carbon nanotube film is studied by means of first-principles calculations. Experiments show that CNT films are responsive to O-3 with a decrease of the resistance similar to that observed for NO2 Our theoretical results suggest the interaction to be pretty strong, as shown by a relatively short equilibrium molecule-tube distance, as well as by an appreciable binding energy and charge transfer from the tube to the adsorbed molecule. The analysis of the density of states shows that a peak in proximity of the nanotube Fermi level is induced by the ozone adsorption. This effect enhances the p-type character of the nanotube and, therefore, the conductivity of the whole film increases, in excellent consistency with the experimentally observed resistance decrease upon O-3 exposure. (C) 2004 American Vacuum Society.